Cerebral β-amyloidosis was found in 16/18 marmosets aged <10 yrs and 8/9 marmosets aged >10 yrs, injected intracerebrally with human or marmoset brain homogenate containing β-amyloid 1–8 years previously. It was found in only 2/12 marmosets aged <10 yrs and 1/15 marmosets aged >10 yrs, injected with synthetic Aβ-peptides, CSF, or brain tissue which did not contain β-amyloid. Cerebral β-amyloidosis was found in 0/11 uninjected marmosets aged <10 yrs and in 5/29 uninjected marmosets aged >10 yrs. The β-amyloidosis comprised small and large vessel angiopathy and some plaques throughout cortex and was qualitatively similar in injected marmosets and, when present, in uninjected marmosets. Of those injected marmosets which were positive, the amount of β-amyloidosis was unrelated to age or incubation times but the 3 injected marmosets without β-amyloidosis had incubation times of <3.5 years. We conclude that β-amyloid, or associated factors, can initiate or accelerate the process of cerebral amyloidosis in primates.